Telecommunication transmission systems utilizing fiber optic technology have surpassed wire based systems as the industry standard because of their desirable characteristics. For example, systems employing fiber optic waveguides are capable of providing much higher bandwidth than wire based systems, they are relatively immune to electromagnetic interference, and they are more secure than their wire based counterparts. Furthermore, components such as optical fiber amplifiers are overshadowing older style repeaters and regenerators due to their demonstrated advantages.
When optical fiber transmission systems were first introduced, they exhibited bandwidth capabilities that easily met contemporary requirements.
Present day bandwidth requirements, however, have increased dramatically and present new challenges over low bandwidth systems such as dispersion control.
As is well known, optical fibers exhibit dispersion; that is, different wavelengths of a signal carried by a fiber propagate at different velocities through the fiber. While dispersion effects of a low bandwidth signal may be relatively insignificant, signal dispersion can be a limiting factor as the bandwidth of the signal increases. For example, modern optical fiber transmission systems can be provided for transmitting data at rates of more than 2.5 gigabits per second. Dispersion compensation is often necessary for reliable, error free transmission at data rates exceeding 2.5 gigabits per second. One method of compensating for dispersion in long haul fiber optic transmission systems is to provide a predetermined length of dispersion compensating fiber in the system. If the dispersion characteristics of a section of a transmission system are known, then a suitable length of dispersion compensating fiber can be provided to reduce or eliminate the overall system dispersion. Such a teaching is set forth in Antos U.S. Pat. No. 5,361,234, granted on Nov. 1, 1994, which is incorporated herein by reference.
As is known, standard transmission fiber is designed for minimum attenuation for signals in the 1550 nm band transmission window, and for minimum dispersion in the 1300 nm band window. Dispersion effects therefore become an issue since most transmission occurs in the 1550 nm window. New long haul fiber transmission systems can be designed to incorporate dispersion shifted fiber to control dispersion at the longer transmission wavelengths. However, millions of kilometers of fiber optic transmission lines are already installed wherein the fiber is minimized for dispersion in the 1300 nm window. Transmission signals in the 1550 nm band therefore exhibit dispersion in amounts that are large enough to require compensation. Moreover, as the transmission signal data rate increases from, say, 2.5 Gbit/s to very high data rates such as 40 Gbit/s, for example, the amount of tolerable dispersion in the system decreases, making the tunability of dispersion compensation a desirable feature. Heretofore, no convenient method has existed for either ad hoc in-field compensation of, or, in-field adjustable tuning of, dispersion in a fiber optic transmission system.
Another inherent characteristic of fiber optical transmission systems is signal attenuation due to loss mechanisms in optical fiber waveguides. In fact, minimization of dispersion and attenuation are two of the chief design challenges associated with both new and existing fiber transmission systems. Due to fiber attenuation, signal regenerators in general and fiber optical amplifiers in particular, are integral components of fiber transmission systems. In fact, fiber optical amplifiers are typically present either alone or in combination at the beginning and end of the system, respectively, as power and pre-amp, and intermediate thereof as an in-line amplifier. Contemporary fiber amplifiers include a fiber waveguide that is doped with a rare earth element (gain fiber) such as erbium, for example. The gain fiber is pumped by an excitation source having a wavelength less than the principal wavelength of the communication signal carried by the fiber. Both the pump and signal wavelengths propagate along the same fiber path. Additional lengths of gain fiber can be added to the signal transmission path to provide further amplification of the communication signals. For instance, depending upon the components that are upstream and downstream of a fiber amplifier, and their distances from the amplifier, a power amplifier could be utilized as an in-line amplifier by adding an additional length of gain fiber to the power amplifier gain fiber. Likewise, additional lengths of doped fiber could be added to the system wherein they could provide a filtering effect on one or more wavelengths in the signal wavelength band for gain spectrum shaping or gain equalization which is important in WDM applications, depending upon the existence or depletion of pump signal in those fibers. Such a teaching is set forth, for example, in Hall U.S. Pat. No. 5,131,069. However, as in the case of dispersion adjustment and tuning, heretofore no convenient method has existed for the ad hoc, in-field adjustment or tuning of communication signal gain or signal and/or pump filtering in a fiber optic transmission system.
A further limitation of amplifier component or gain block modules used in presently installed systems involves dynamic gain tilt in multi-channel applications; i.e., a change in the gain spectrum with changes in component or module operating conditions. An amplifier can be designed to provide some optimum level of gain uniformity over a given operation band, but this can generally only be achieved for a specific set of signal input powers and pump powers. Therefore, if deployment requirements include a change in signal gain, the gain uniformity will degrade as the overall gain spectrum changes. Consider, for example, an optimized multi-channel fiber transmission system including at least two amplifier stages typically separated by a distance of about 90 km. Over this length of fiber, typical signal loss due to attenuation and other factors will be about minus 23 dB. Each amplifier stage is further limited to a power output of about 8 dBm/channel because of induced non linear effects in the fiber when output power is greater than about 8 dBm. Such non linear effects include, for example, self- and cross-phase modulation and four-wave mixing phenomena which are highly detrimental to low error signal reception. By simple arithmetic then, the input power at the next downstream amplifier will be minus 15 dBm. For these input, loss and output power values, an average saturated inversion level in a pumped, doped amplifying fiber can be maintained to provide a relatively flat gain spectrum from about 1536 nm to 1560 nm when the fiber is doped with erbium.
Consider now the scenario in which the distance between gain stages must be reduced to, say, 50 km due to a constraint on amplifier placement (e.g., a mountain or lake). A typical attenuation over this reduced distance would be minus 13 dB, resulting in a signal power at the input of the next downstream gain stage equal to minus 5 dBm. However, unless the gain of the amplifier stage can be reduced such that the power output remains at about 8 dBm, by, for example, reducing the drive current to the doped fiber pumping source, induced non linear effects in the fiber again become a problem. It will be appreciated, though, that reducing the gain in the amplifier stage by decreasing the average inversion level will lead to dynamic gain tilt as described above, which must be minimized for multi-channel system operation. A solution to this problem not heretofore available is presented by the disclosed invention by providing a gain platform or optical amplifier having a selectively switchable route through one of at least two differing lengths of doped fiber whereby the amplifier output power can be maintained at the desired level substantially independently of the higher or lower input signal power to maintain the desired gain uniformity over the desired operating band.
It is therefore an object of this invention to provide a fiber optic device having switchable characteristics for field selecting variable amounts of optical signal gain, signal filtering and/or signal dispersion, in a fiber optic transmission system and/or an amplifying component thereof.
This invention has as a further object to provide a component of a fiber transmission system, such as a fiber optical amplifier, or active or passive gain platform, that allows for variable, field selectable levels of optical signal gain, signal filtering and/or signal dispersion by selectively routing a signal through different fiber lengths having similar or different characteristics.
It is a still further object of this invention to provide a fiber optic transmission system having variable, field switchable amounts of optical signal gain, signal filtering and/or signal dispersion.